1. Field of the Invention
The invention relates to a mercury lamp of the short arc type which is used for a semiconductor exposure device and which exhibits only minor changes of the radiation efficiency.
2. Description of the Related Art
In the exposure process in the production of semiconductors, a mercury lamp of the short arc type has recently been used which emits UV light with a primary wavelength of 365 nm (hereinafter called "i-line light"). Since the degree of integration of integrated semiconductor circuits increases each year, accordingly there is a greater and greater demand for image resolution during exposure. Furthermore, due to the increase in the size of the exposure surface as a result of enlargement of the wafer aperture, or due to the modified illumination technique which is used to achieve high image resolution, there is a demand for an increase in the amount of UV light radiation from the light source.
An increase of irradiance of the i-line light can be achieved by increasing the electrical input power for the lamp. However, an increase of electrical input power causes an increase in the inflowing amount of energy to the electrodes, and thus, a rise in the electrode temperature; this increases vaporization of the electrode material and the impurities contained therein with resultant damage to the electrodes and blackening of the bulb. To reduce the electrode temperature, there is a process for increasing the electrode dimensions. This increase of the electrode dimensions, however, causes an increase in the amount of carbon which the electrodes contain as impurities.
Generally, gaseous contamination, such as oxygen or the like, which is present in the bulb reacts with the tungsten which comprises the electrodes. This yields tungsten oxide or tungsten as a compound with a low melting point. It is well recognized that this tungsten oxide or the like causes deformation of the electrodes in itself and vaporization of the electrode material because it has a lower melting point than tungsten.
The gaseous contamination in the bulb was therefore eliminated in a conventional mercury lamp by using titanium, zirconium, tantalum and the like as a getter. In this way, gaseous contamination, such as oxygen or the like, could be advantageously eliminated by using the above described getter. However, with respect to the carbon present in the bulb, it has not yet been sufficiently clarified how it behaves in a high temperature range of at least a thousand and some hundred degrees Celsius and what capacity the above described getter has.
Here, the tungsten anode conventionally contains a few ppm of carbon. During operation of the lamp, the anode is exposed to a high temperature. Therefore, there are cases in which the carbon contained in this anode sprays in the emission space in the bulb during lamp operation. When the sprayed carbon is adsorbed on the area of the anode or the cathode of tungsten which comes into contact with the arc, tungsten carbide is formed which has a low melting point. Here, vaporization from this region is accelerated.
On the other hand, in the cathode, conventionally thorium oxide with a proportion of a few % by weight is dispersed to simplify electron emission. Furthermore, on the outer surface of the cathode, a layer of tungsten carbide (carbide layer) is formed. This is because the thorium oxide contained in the cathode is reduced and becomes metallic thorium, and in this way, electron emission is simplified.
If, however, the layer of tungsten carbide located on the outer surface of the cathode is exposed to a high temperature during operation of the lamp, carbon from this layer of tungsten carbide vaporizes. This carbon floats in the emission space in the bulb and can be deposited on the inner surface of the bulb. Furthermore, if this floating carbon is adsorbed on the tip of the cathode or anode, on which tungsten comes into direct contact with the arc, vaporization at this adsorption site is accelerated even more.
This is because the tungsten carbide has a lower melting point than tungsten and therefore vaporizes more rapidly. This vaporization of the material which comprises the electrodes on the electrode tips leads to considerable attenuation of the amount of radiated light.